Method and apparatus for controlling handover in wireless communication system

ABSTRACT

A method and apparatus are provided that controls the handover between DUs in an eNB including a CU and a DU. The method and system fuses 5G communication systems with IoT technology to transmission data at a high rate after 4G systems. The communication method and system is applied to intelligent services, based on 5G communication technology and IoT related technology. The method includes transmitting, to a source DU of the base station, a first message related to a handover; receiving, from the source DU, a second message for downlink data delivery information related to a PDU transmitted from the source DU to a terminal; receiving, from a target DU of the base station, a third message based on a random access procedure of the terminal toward the target DU; and transmitting, to the target DU, downlink data based on the downlink data delivery information after receiving the third message.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Continuation of U.S. patent application Ser. No.16/421,816, which was filed in the U.S. Patent and Trademark Office onMay 24, 2019, which is a Continuation of U.S. patent application Ser.No. 15/593,829, which was filed in the U.S. Patent and Trademark Officeon May 12, 2017, and issued as U.S. Pat. No. 10,306,526 on May 28, 2019,the entire content of each of which is incorporated herein by reference.

BACKGROUND 1. Field of the Invention

Embodiments of the present disclosure relate to a method and apparatusfor controlling handover in a wireless communication system. Moreparticularly, embodiments of the present disclosure relate to a methodand apparatus for controlling the handover between distributed units(DUs) (inter-DU handover) in an evolved node B (eNB) including a centralunit (CU) and a distributed unit (DU).

2. Description of Related Art

In order to meet the increase in the demand for wireless data trafficafter the commercialization of 4G communication systems, considerableeffort has been made to develop pre-5G communication systems or improved5G communication systems. This is one reason why ‘5G communicationsystems’ or ‘pre-5G communication systems’ are called ‘beyond 4G networkcommunication systems’ or ‘post LTE systems.’ In order to achieve a highdata transmission rate, 5G communication systems are being developed tobe implemented in a band of extremely high frequency, or millimeter wave(mmWave), e.g., a band of 60 GHz. In order to reduce the occurrence ofstray electric waves in a band of extremely high frequency energy and toincrease the transmission distance of electric waves in 5G communicationsystems, various technologies being explored, for example: beamforming,massive MIMO, Full Dimensional MIMO (FD-MIMO), array antennas, analogbeam-forming, large scale antennas, etc. In order to improve systemnetworks for 5G communication systems, various technologies have beendeveloped, e.g., evolved small cell, advanced small cell, cloud radioaccess network (cloud RAN), ultra-dense network, Device to Devicecommunication (D2D), wireless backhaul, moving network, cooperativecommunication, Coordinated Multi-Points (CoMP), interferencecancellation, etc. In addition, for 5G communication systems, othertechnologies have been developed, e.g., Hybrid FSK and QAM Modulation(FQAM) and Sliding Window Superposition Coding (SWSC), as AdvancedCoding Modulation (ACM), Filter Bank Multi Carrier (FBMC),non-orthogonal multiple access (NOMA), sparse code multiple access(SCMA), etc.

The Internet has evolved from a human-based connection network, wherehumans create and consume information, to the Internet of Things (IoT)where distributed configurations, such as objects, exchange informationwith each other to process the information. The technology related tothe IoT is starting to be combined with, for example, a technology forprocessing big data through connection with a cloud server, and this iscalled an Internet of Everything (IoE) technology. In order to manifestthe IoT, various technical components are required, such as, a sensingtechnology, wired/wireless communication and network infra technology, aservice interfacing technology, a security technology, etc. In recentyears, a sensor network for connecting objects, Machine to Machine(M2M), Machine Type Communication (MTC), etc. have been researched.Under the IoT environment, intelligent Internet Technology (IT) servicesmay be provided to collect and analyze data obtained from objectsconnected to each other and thus to create new value for human life. Asexisting information technologies are fused and combined with variousindustries, the IoT may also be applied within various fields, such as:smart homes, smart buildings, smart cities, smart cars or connectedcars, smart grids, health care, smart home appliances, high qualitymedical services, etc.

To this end, various attempts have been made to apply 5G communicationsystems to the IoT. For example, various technologies related to sensornetworks, Machine to Machine (M2M), Machine Type Communication (MTC),etc., have been implemented by beam-forming, MIMO, array antenna, etc.,as 5G communication technology. The application of the cloud RAN as abig data processing technology described above may be an example of ahybrid of 5G technology and IoT technology.

In communication systems described above, a number of nodes may performoperations of an eNB. More specifically, nodes may perform operationsrelated to an eNB, based on central units (CUs) and distributed units(DUs). Therefore, communication systems need to set a signaling methodbetween a CU and a DU in order to smoothly provide services.

SUMMARY

The present invention has been made to address the above problems anddisadvantages, and to provide at least the advantages described below.Accordingly, embodiments of the present disclosure provide: a method ofsmoothly performing the handover between distributed units (DUs)(inter-DU handover) by signaling with a central unit (CU) in acommunication providing services through CUs and DUs; and an apparatusadapted to the method.

In accordance with an aspect of the present invention, a method isprovided for a CU of a base station. The method includes transmitting,to a source DU of the base station, a first message related to ahandover; receiving, from the source DU, a second message for downlinkdata delivery information related to a packet data unit (PDU)transmitted from the source DU to a terminal; receiving, from a targetDU of the base station, a third message based on a random accessprocedure of the terminal toward the target DU; and in response toreceiving the third message, transmitting downlink data to the target DUbased on the downlink data delivery information, before receiving afourth message associated with a radio resource control (RRC)reconfiguration complete based on the random access procedure of theterminal from the target DU.

In accordance with another aspect of the present invention, a method isprovided for a source DU of a base station. The method includesreceiving, from a CU of the base station, a first message related to ahandover; and transmitting, to the CU, a second message for downlinkdata delivery information related to a packet data unit (PDU)transmitted to a terminal. A third message is transmitted from a targetDU of the base station to the CU based on a random access procedure ofthe terminal toward the target DU. In response to a reception of thethird message at the CU, downlink data is transmitted from the CU to thetarget DU based on the downlink data delivery information, before afourth message associated with a radio resource control (RRC)reconfiguration complete based on the random access procedure of theterminal is received at the CU.

In accordance with another aspect of the present invention, a method isprovided for a target DU of a base station. The method includestransmitting, to a CU of the base station, if a first message related toa handover is transmitted from the CU of the base station to a source DUof the base station and a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal is transmitted from the source DU to the CU, athird message based on a random access procedure of the terminal towardthe target DU; and in response to a reception of the third message atthe CU, receiving downlink data from the CU based on the downlink datadelivery information, before transmitting a fourth message associatedwith a radio resource control (RRC) reconfiguration complete based onthe random access procedure of the terminal to the CU.

In accordance with another aspect of the present invention, a CU of abase station is provided. The CU includes a transceiver; and acontroller, which is configured to control the transceiver to transmit,to a source DU of the base station, a first message related to ahandover, receive, from the source DU, a second message for downlinkdata delivery information related to a packet data unit (PDU)transmitted from the source DU to a terminal, receive, from a target DUof the base station, a third message based on a random access procedureof the terminal toward the target DU, and in response to receiving thethird message, transmit downlink data to the target DU based on thedownlink data delivery information, before receiving a fourth messageassociated with a radio resource control (RRC) reconfiguration completebased on the random access procedure of the terminal from the target DU.

In accordance with another aspect of the present invention, a source DUof a base station is provided. The source DU includes a transceiver; anda controller, which is configured to control the transceiver to receive,from a CU of the base station, a first message related to a handover,and transmit, to the CU, a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted to aterminal. A third message is transmitted from a target DU of the basestation to the CU based on a random access procedure of the terminaltoward the target DU. In response to a reception of the third message atthe CU, downlink data is transmitted from the CU to the target DU, basedon the downlink data delivery information, before a fourth messageassociated with a radio resource control (RRC) reconfiguration completebased on the random access procedure of the terminal is received at theCU.

In accordance with another aspect of the present invention, a target DUof a base station is provided. The target DU includes a transceiver; anda controller, which is configured to control the transceiver totransmit, to a CU of the base station, if a first message related to ahandover is transmitted from the CU of the base station to a source DUof the base station and a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal is transmitted from the source DU to the CU, athird message based on a random access procedure of the terminal towardthe target DU, and in response to a reception of the third message atthe CU, receive downlink data from the CU based on the downlink datadelivery information, before transmitting a fourth message associatedwith a radio resource control (RRC) reconfiguration complete based onthe random access procedure of the terminal to the CU.

In accordance with another aspect of the present invention, a method isprovided for a CU of a base station. The method includes transmitting,to a source DU of the base station, a first message related to ahandover; receiving, from the source DU, a second message for downlinkdata delivery information related to a packet data unit (PDU)transmitted from the source DU to a terminal; receiving, from a targetDU of the base station, a third message based on a random accessprocedure of the terminal toward the target DU; and transmitting, to thetarget DU, downlink data based on the downlink data delivery informationafter receiving the third message.

In accordance with another aspect of the present invention, a method isprovided for a target DU of a base station. The method includestransmitting, to a CU of the base station, if a first message related toa handover is transmitted from the CU of the base station to a source DUof the base station and a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal is transmitted from the source DU to the CU, athird message based on a random access procedure of the terminal towardthe target DU; and receiving, from the CU, downlink data based on thedownlink data delivery information after a reception of the thirdmessage at the CU.

In accordance with another aspect of the present invention, a CU of abase station is provided. The CU includes a transceiver; and acontroller, which is configured to control the transceiver to transmit,to a DU of the base station, a first message related to a handover,receive, from the source DU, a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal, receive, from a target DU of the base station,a third message based on a random access procedure of the terminaltoward the target DU, and transmit, to the target DU, downlink databased on the downlink data delivery information after a reception of thethird message.

In accordance with another aspect of the present invention, a target DUof a base station is provided. The target DU includes a transceiver; anda controller, which is configured to control the transceiver totransmit, to a CU of the base station, if a first message related to ahandover is transmitted from the CU of the base station to a source DUof the base station and a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal is transmitted from the source DU to the CU, athird message based on a random access procedure of the terminal towardthe target DU, and receive, from the CU, downlink data based on thedownlink data delivery information after a reception of the thirdmessage at the CU.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the inventionwill be more apparent from the following detailed description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a flow diagram that describes a method of performing thetransmission/reception of signals between a base station (eNB) andterminal (or user equipment (UE)) when performing inter-cell handover ofUE according to embodiments of the present disclosure;

FIG. 2 is a diagram showing the architecture of layers of a CU and a DUaccording to embodiments of the present disclosure;

FIG. 3 is a flow diagram that describes a signaling method when inter-DUhandover of UE is performed according to embodiments of the presentdisclosure;

FIG. 4 is a diagram showing a packet data unit (PDU) transmitted whenhandover of UE is performed according to embodiments of the presentdisclosure;

FIG. 5 is a block diagram of a DU according to embodiments of thepresent disclosure; and

FIG. 6 is a block diagram of a CU according to embodiments of thepresent disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the present invention are described indetail with reference to the accompanying drawings.

In the embodiments, detailed descriptions of the technical content whichis well-known and is not directly related to the embodiments of thepresent disclosure are omitted to avoid obscuring the subject matter ofthe invention, thereby highlighting the subject matter of the inventionmore clearly.

Similarly, the drawings are not necessarily to scale and certainfeatures may be exaggerated, omitted, or simplified in order to betterillustrate and explain the invention. The same reference numbers areused throughout the drawings to refer to the same or correspondingparts.

The features and advantages of the invention and the methods toaccomplish the objectives of the invention will become more apparentfrom the following detailed description and the accompanying drawings.Although embodiments of the invention have been described in detail, itshould be understood that many variations and modifications of the basicinventive concept herein described, which may be apparent to thoseskilled in the art, will still fall within the spirit and scope of theexemplary embodiments of the invention as defined in the appendedclaims. The same reference numbers are used throughout the drawings torefer to the same parts.

In addition, it should be understood that the blocks in the signalflowcharts and the combinations in the flowcharts can be performed viacomputer programming instructions. These computer programminginstructions can be installed to processors of data processing equipmentthat can be programmed, special computers, or universal computers. Theinstructions, performed via the processors of data processing equipmentor the computers, can create means that perform functions described inblocks of the flow charts. In order to implement functions in aparticular mode, the computer programming instructions can be stored ina computer available memory or computer readable memory that can supportcomputers or data processing equipment that can be programmed.Therefore, the instructions, stored in the computer available memory orcomputer readable memory, can be installed to the products, and performthe functions described in the block(s) of the flow charts. In addition,since the computer programming instructions can also be installed tocomputers or data processing equipment that can be programmed, they cancreate computer-executable processes as a series of operations areperformed therein, described in the block(s) of the flow charts therein.

The blocks of the flow charts refer to part of codes, segments ormodules that include one or more executable instructions to perform oneor more logic functions. It should be noted that the functions describedin the blocks of the flow charts may be performed in a different orderfrom the embodiments. For example, the functions described in twoadjacent blocks may be performed at the same time or in reverse order.

In the embodiments, the terminology, component ‘˜ unit,’ refers to asoftware element or a hardware element such as an FPGA, an ASIC, etc.,and performs a corresponding function. It should be, however, understoodthat the component ‘˜unit’ is not limited to a software or hardwareelement. The component ‘˜unit’ may be implemented in storage media thatcan be designated by addresses. The component ‘˜unit’ may also beconfigured to regenerate one or more processors. For example, thecomponent ‘˜unit’ may include various types of elements (e.g., softwareelements, object-oriented software elements, class elements, taskelements, etc.), processes, functions, attributes, procedures,sub-routines, segments of program codes, drivers, firmware, micro-codes,circuit, data, data base, data structures, tables, arrays, andvariables. Functions provided by elements and the components ‘˜units’may be formed by combining the small number of elements and components‘˜units’ or may be divided into additional elements and components‘˜units.’ In addition, elements and components ‘˜units’ may also beimplemented to regenerate one or more CPUs in devices or securitymulti-cards.

Embodiments of the present disclosure provide a control method ofsupporting efficient UE mobility in an eNB employing the architecture ofseparation of a central unit (CU) and a distributed unit (DU). Inembodiments, a CU and a DU may be referred to as units included inindependent eNBs or the same eNB, respectively. Alternatively, inanother embodiment, a CU and a DU may be configured with differentnetwork entities, respectively.

FIG. 1 is a flow diagram that describes a method of performing thetransmission/reception of signals between a base station (evolved nodeB, eNB) and terminal (user equipment, UE) when performing inter-cellhandover of UE according to embodiments of the present disclosure.

With reference to FIG. 1, UE 102 is capable of transmitting/receivingsignals to/from an eNB 104. More specifically, UE 102 is capable oftransmitting/receiving signals for handover between cells controlled byan eNB 104.

UE 102 is capable of transmitting a measurement report to the eNB 104 inoperation 110, in the embodiment, the measurement report may betransmitted via a radio resource control (RRC) message.

The eNB 104 is capable of determining whether it needs to perform thehandover of UE 102, based on the received measurement report inoperation 115.

If the eNB 104 ascertains that it needs to perform the handover of UE102 in operation 115, the eNB 104 is capable of transmitting an RRCConnection Reconfiguration message to the UE 102 in operation 120. Morespecifically, the eNB 104 performs a preparation process for handoverand then transmits an RRC Connection Reconfiguration message to the UE102.

The eNB 104 is capable of stopping the transmission/reception of databetween a radio link control (RLC) layer and a media access control(MAC) layer in a source cell, in response to the determination of thehandover, in operation 125.

The eNB 104 is capable of performing operations related to thetransmission of RLC/MAC layer data to UE, via a target cell, in responseto the handover of UE 102 in operation 130.

The UE 102 is capable of performing a random access procedure includingthe transmission of a random access preamble to the eNB 104 in operation135. In this case, the preamble may be a dedicated preamble.

The eNB 104 is capable of receiving an RRC connection reconfigurationcomplete message from the UE 102 in operation 140, and then transmitsactual data to the UE 102. If the eNB 104 receives the message, it iscapable of transmitting downlink data from the PDCP layer to the UE 102via a target cell.

As described above, since the inter-cell handover operation is performedin the same eNB where RRC/PDCP/RLC/MAC layers exist, a source RLC/MAC iscapable of detecting a timing when the data transmission needs to bestopped, and requesting, right after that, the resumption oftransmission from a target RLC/MAC, thereby reducing the occurrence ofthe transmission delay or the data redundancy transmission.

FIG. 2 is a diagram showing the architecture of layers of a CU and a DUaccording to embodiments of the present disclosure.

With reference to FIG. 2, in an eNB employing the architecture ofseparation of a central unit (CU) and a distributed unit, thearchitecture of connection of multiple DUs located below the same CU,with the protocol stack by layers, is shown.

The eNB is capable of including a CU 210 and DUs 220 and 230. Each ofthe units is capable of performing operations of the eNB. In theembodiment, each unit may be referred to as an independent eNB.

The CU 210 is capable of performing operations of RRC and PDCP layers,and transmitting/receiving signals to/from a number of DUs 220 and 230.

The DUs 220 and 230 are connected to the CU 210 and capable of receivingPDU from the PDCP layer. The DUs 220 and 230 are capable of performingoperations of RLC, MAC, physical (PHY) and radio frequency (RF) layers.

It should be understood that the CU 210 and DUs 220 and 230 may also beimplemented in different configuration. For example, the embodiment maybe modified in such a way that radio-related layers of the DUs 220 and230 may be separated to other nodes. It should be understood that thepresent invention is not limited to the embodiment and have variousalternations and modifications without departing from the spirit of thepresent invention.

The CU 210 may be modified in such a way as to further include otherlayers or remove part of the layers; however, if the modificationtransmits/receives signals to/from a node performing operations relatedto the PDCP layer and operations of the RLC layer, embodiments relatedto the CU 210 of the present disclosure may also apply to themodification.

The DUs 220 and 230 may be modified in such a way as to further includeother layers or remove part of the layers; however, if the modificationtransmits/receives signals to/from a node performing operations relatedto the RLC layer and operations of the PDCP layer, embodiments relatedto the DUs 220 and 230 of the present disclosure may also apply to themodification.

From the viewpoint of a device, nodes to which the embodiments areapplied may also be called CU or DU.

As described above, the CU 210 is connected to a number of DUs 220 and230. In order to perform the handover between DUs (inter DU handover), asignaling method needs to be set between the CU 210 and the DUs 220 and230. More specifically, unlike the existing handover by a single eNB,since an eNB is separated into two network entities, CU and DU, andRLC/MAC layers are arranged in DUs, a lower layer informs a higher layerlocated in the CU of start and end timings of the packet transmission,and shares an end timing of the packet transmission in a source RLC/MACwith a target DU via the CU, and thus there may be a need to resume thedata transmission fast.

If a handover procedure for the architecture of a single eNB is appliedin order to perform the operations described above, lower layers of asource DU, RLC/MAC, do not know timings to end the handover procedureand to stop the packet transmission. In particular, although UE contexthas already moved from the source DU to a target DU, the source DU mayperform the redundancy transmission of data, which is not necessary, andmay also delay the timing to resume the data transmission.

Therefore, the CU informs a DU, in which an RLC/MAC layer is located, ofthe handover start timing, so that the PDU transmission in the sourceDU, which is unnecessary, can be stopped at an appropriate timing andthe PDU transmission adapted to the target DU can be resumed, using PDUSN information shared between source/target DUs, thereby reducing atransmission delay of user data when the inter-DU handover is performed.In the embodiment based on the operation described above, if UE,connected to an eNB employing the architecture of CU-DU separation,performs the handover between different DUs in the same CU, the UE iscapable of providing: the inter-DU handover for fast PDU retransmit; andalso an early forwarding function to a target-DU for a PDCP PDU whichhas not been transmitted when a handover procedure is processed becauseof the inter-DU connection architecture in the CU.

Messages, which can be transmitted between CU and DU, are as follows.

1. Handover Start Indication (transmitted from CU to Source DU)

-   -   Information to stop the DL data transmission of Source DU    -   Information to trigger the transmission of Delivery Status to CU

2. Delivery Status (transmitted from source DU to CU)

-   -   Information to notify CU of information regarding DL data that        has not been transmitted

3. Dedicated RACH Preamble Detection Indication (transmitted from targetDU to CU)

-   -   Information to indicate a user plane function to prepare to        forward user data to target DU    -   Information to early forward a DL PDCP packet, including PDCP        PDU which has not been transmitted, to a target DU

The embodiment is capable of performing the handover between DUsconnected to the same CU, employing the messages described above.

FIG. 3 is a flow diagram that describes a signaling method when inter-DUhandover of UE is performed according to embodiments of the presentdisclosure.

With reference to FIG. 3, UE 302 is capable of performing the handoverfrom a source DU 304 connected to CU 308 to a target DU 306. To thisend, the UE 302 starts a signaling procedure.

UE 302 is capable of transmitting, to the source DU 304, a MeasurementReport including channel measurement information in operation 310.

The source DU 304 is capable of forwarding the received MeasurementReport to the CU 308 in operation 312. More specifically, the source DU304 forwards a Measurement Report to the CU 308 via an RRC TRANSFERmessage.

The CU 308 is capable of determining whether it performs the inter-DUhandover, based on the received information in operation 314. In theembodiment, the CU 308 is capable of determining to perform the handoverfrom the source DU 304 to the target DU 306.

The CU 308 transmits a message, UE CONTEXT SETUP REQUEST, to the targetDU 306, thereby transmitting a handover indicator and RB information forthe handover call setup in operation 316.

The target DU 306 is capable of transmitting a message, UE CONTEXT SETUPRESPONSE, to the CU in response to the response in operation 318. In theembodiment, the UE CONTEXT SETUP RESPONSE message may contain at leastone of the following: C-RNTI and dedicated RACH information. In theembodiment, the CU 308 is capable of setting up a new UE context forhandover to the target DU 306.

After setting up a new UE context to the target DU 306, the CU 308 iscapable of transmitting, to the UE, an RRC Connection Reconfigurationmessage, via the RRC Transfer, through the source DU 304, in operations320 and 322. The RRC Connection Reconfiguration message may contain atleast one of the following: MAC/PHY information and RB for connecting tothe target DU 306. In the embodiment, the RRC Connection Reconfigurationmessage may contain information related to the setup UE context.

The CU 308 is capable of transmitting a message indicating the handoverstart to the source DU 304 in operation 324. More specifically, the CU308 is capable of transmitting a message indicating the handover start,HANDOVER START INDICATION including Delivery Status Indicator, to thesource DU 304. In the embodiment, the Delivery Status Indicator maycontain PDCP SN information regarding the first missing acknowledgementright after the last PDCP PDU, which is transmitted via the PDCPtransmission and ascertained by the CU 308. Since a correspondingmessage is transmitted via a GTP-U, the GTP header contains GTP SN.After that, when the DU receives the HANDOVER START INDICATION message,the DU may obtain an RLC SN referring to the GTP SN value. In theembodiment, the HANDOVER START INDICATION message may containinformation as in the following table 1.

TABLE 1 Description IE Name CU UE ID UE identification of CU DU UE ID UEidentification of DU RB list >>RB ID Radio bearer identification foreach UE context >>Delivery Status First missing PDCP sequence numberwhich has Indicator not been received acknowledgement This informationcan be provided by PDCP layer of CU

The source DU 304 is capable of stopping the DL data transmission inresponse to the reception of the HANDOVER START INDICATION message inoperation 326. More specifically, the source DU 304 is capable ofstopping the DL data transmission, based on information included in theHANDOVER START INDICATION.

The source DU 304 is capable of transmitting, to the CU 308, a messageincluding information related to data transmitted to UE, DELIVERYSTATUS, in operation 328. More specifically, the source DU 304 iscapable of transmitting, to the CU 308, a DELIVERY STATUS messageincluding information regarding the transmission of DL RLC PDU, In thiscase, the source DU 304 is capable of transmitting, to the CU 308,information regarding the RLC complete transmission which has beencompleted according to the reception of an acknowledgement in responseto the DL RLC PDU and information regarding the RLC incompletetransmission which has not been completed because the acknowledgementhas not been received. In the embodiment, the DELIVERY STATUS messagemay contain information as in the following table 2.

TABLE 2 Description IE Name CU UE ID UE identification of CU DU UE ID UEidentification of DU RB list >>RB ID Radio bearer identification foreach UE context >>Delivery Status RLC sequence number which has beenreceived Report acknowledgement, Missing RLC sequence number which hasnot been received acknowledgement. This information can be provided byRLC layer of DU

The UE 302 is capable of performing a random access procedure with thetarget DU 306 in operation 330. More specifically, the UE is capable ofperforming the random access to the target DU 306, using a dedicatedpreamble.

The target DU 306 is capable of transmitting, to the CU 308, a messagecontaining information notifying the UE random access in operation 332.More specifically, the target DU 306 is capable of transmitting amessage, DEDICATED RACH PREAMBLE DETECTION INDICATION, to the CU 308.

The CU 308 is capable of resuming the downlink data transmission to theUE 302 in operations 334 and 336. More specifically, the CU 308 iscapable of resuming the downlink data transmission, based on informationreceived in operations 328 and 332. In an embodiment, if the CU 308receives a DELIVERY STATUS message from the source DU 304 andsuccessfully performs the RACH procedure to the target DU 306 of the UE302, the CU 308 is capable of performing the downlink data transmissionbased on the result. In this case, the CU 308 is capable of forwardingdownlink packets to the target DU 306, via the DL Data Transfer, basedon information related to packets transmitted from the source DU 304.The target DU 306 is capable of immediately transmitting the downlinkpackets to the UE 302 since the target DU 306 has performed a RACHprocedure with the UE 302. As such, the embodiment resumes thetransmission of data that the source DU 304 has been transmitting beforethe RRC connection reconfiguration is completed, thereby rapidlytransmitting data. In an embodiment, the CU 308 may forward informationrelated to downlink PDU to be transmitted to the target DU 306, based oninformation obtained in operation 328, in operation 334. In theembodiment, the DL PDCP PDU, transmitted to UE 302 as in operations 334and 336, may include PDUs which are incompletely transmitted from thesource DU 304, and thus this makes it possible tc fast transmission theDL PDCP PDU right after the inter-DU handover.

The UE 302 is capable of transmitting an RRCConnReconfComplete messageto the target DU 306 based on a procedure after the random access inoperation 338, and the target DU 306 is capable of transmitting themessage to the CU 308 in operation 340.

If the UE 302 has established the connection with the target DU 306, theCU 308 is capable of transmitting a message for releasing UE context, UEContext Release Request, to the source DU 304 in operation 342. Thesource DU 304 is capable of transmitting the response message to the CU308 in operation 346. The embodiment is capable of transmitting the UEContext Release Response message to the CU, thereby informing the CU ofa condition as to whether the UE context release procedure issuccessfully performed.

If the CU 308 receives a PDCP status from the UE, via STATUS PDU, at atiming, the CU 308 processes packets which have not been transmittedduring the fast resume operation and the DL PDCP PDU which has beenreceived from a Core Network but is buffered in the CU 308 in operations348 and 350. The CU 308 is capable of transmitting the DL PDCP PDU tothe target DU 306 in operation 352. The target DU 306 is capable offorwarding the DL PDCP PDU to the UE in operation 354.

In the embodiment, the message as in operation 332 may be used for theearly Radio Bearer resumption described above. If the message is nottransmitted or is lost, the embodiment may resume the downlink datatransmission after operation 338 or 340.

In the embodiment, after transmitting a HANDOVER START INDICATIONmessage to the source DU 304, if the CU 308 has not received a responseto the DELIVERY STATUS or has received an incorrect response from thesource DU 304, the CU 308 may transmit DL Data after receiving aDEDICATED RACH PREAMBLE DETECTION INDICATION message and an RRCConnection Reconfiguration Complete message in operation 332.

Before the CU 308 receives PDCP PDU-related information that the UE 302received as in operations 348 and 350, the embodiment of the presentdisclosure is capable of resuming the downlink data transmission as inoperations 334, etc., thereby rapidly transmitting data.

FIG. 4 is a diagram showing a packet data unit (PDU) transmitted whenhandover of UE is performed according to embodiments of the presentdisclosure.

With reference to FIG. 4, a source DU starts with a downlink PDU whichis transmitted. In the embodiment, the source DU is capable oftransmitting the downlink PDU to UE. FIG. 4 shows PDU 425 that hasreceived an acknowledgement, PDU 430 that has received a negativeacknowledgement, and PDU 435 that has not received a response after thetransmission.

In the embodiment, the source DU is capable of receiving handover startindication from a CU and stopping the downlink data transmission inresponse to the reception of the handover start indication as indicatedby reference number 415. Therefore, the following PDU 420 may not betransmitted to the UE.

Therefore, the source DU is capable of transmitting, to the CU,information regarding the transmitted PDU. The information regarding thetransmitted PDU may include at least one of the following: informationregarding PDU succeeding in transmission, information regarding PDUfailing in transmission, and information regarding PDU that has notreceived a response.

After receiving the information described above, the CU is capable oftransmitting corresponding information to a target DU and resuming thetransmission of downlink data to be transmitted to the UE.

FIG. 5 is a block diagram of a DU according to embodiments of thepresent disclosure.

With reference to FIG. 5, a DU 500 of embodiments includes a transceiver502, a storage unit 504 and a controller 506.

The transceiver 502 is capable of transmitting/receiving signals to/fromUE and/or CU.

The storage unit 504 is capable of storing at least one of thefollowing: information related to the DU 500 and informationtransmitted/received via the transceiver 502. In the embodiment, thestorage unit 504 is capable of storing context information regarding UEand buffering transmission data.

The controller 506 is capable of controlling operations of the DU 500.The controller 506 is capable of controlling the DU to performoperations related to DUs as described in the embodiments. Thecontroller 506 may include at least one processor.

FIG. 6 is a block diagram of a CU according to embodiments of thepresent disclosure.

With reference to FIG. 6, a CU 600 of embodiments includes a transceiver602, a storage unit 604 and a controller 606.

The transceiver 602 is capable of transmitting/receiving signals to/fromUE and/or DUs.

The storage unit 604 is capable of storing at least one of thefollowing: information related to the CU 600 and informationtransmitted/received via the transceiver 602. In the embodiment, thestorage unit 604 is capable of storing context information regarding UEand buffering transmission data.

The controller 606 is capable of controlling operations of the CU 600.The controller 606 is capable of controlling the CU to performoperations related to CU as described in the embodiments. The controller606 may include at least one processor.

As described above, when an eNB employing the architecture of separationof a central unit (CU) and a distributed unit (DU) performs the handoverbetween distributed units (DUs) (inter-DU handover), embodiments of thepresent disclosure are capable of rapidly transmitting data during thehandover. In addition, when an eNB employing the architecture ofseparation of a CU and a DU performs the handover, embodiments of thepresent disclosure are capable of securing the reliability oftransmission, thereby reducing the signal overhead.

The terms and words used in the description and the drawings are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of the presentdisclosure. Accordingly, it should be apparent to those skilled in theart that the description of various embodiments of the presentdisclosure is provided for illustration purpose only and not for thepurpose of limiting the present disclosure as defined by the appendedclaims and their equivalents.

What is claimed is:
 1. A method by a central unit (CU) of a basestation, the method comprising: transmitting, to a source distributedunit (DU) of the base station, a first message related to a handover;receiving, from the source DU, a second message for downlink datadelivery information related to a packet data unit (PDU) transmittedfrom the source DU to a terminal; receiving, from a target DU of thebase station, a third message based on a random access procedure of theterminal toward the target DU; and in response to receiving the thirdmessage, transmitting downlink data to the target DU based on thedownlink data delivery information, before receiving a fourth messageassociated with a radio resource control (RRC) reconfiguration completebased on the random access procedure of the terminal from the target DU.2. The method of claim 1, wherein the downlink data delivery informationincludes information on unsuccessfully transmitted downlink data to theterminal.
 3. The method of claim 1, wherein the downlink data deliveryinformation includes at least one of information on a successfullydelivered packet or information on a lost packet.
 4. The method of claim1, wherein the first message includes packet data convergence protocol(PDCP) sequence number (SN) information for a first PDCP PDU, which is afirst missing acknowledgement after a last PDCP PDU for whichtransmission has been confirmed.
 5. A method by a source distributedunit (DU) of a base station, the method comprising: receiving, from acentral unit (CU) of the base station, a first message related to ahandover; and transmitting, to the CU, a second message for downlinkdata delivery information related to a packet data unit (PDU)transmitted to a terminal, wherein a third message is transmitted from atarget DU of the base station to the CU based on a random accessprocedure of the terminal toward the target DU, and wherein in responseto a reception of the third message at the CU, downlink data istransmitted from the CU to the target DU based on the downlink datadelivery information, before a fourth message associated with a radioresource control (RRC) reconfiguration complete based on the randomaccess procedure of the terminal is received at the CU.
 6. The method ofclaim 5, wherein the downlink data delivery information includesinformation on unsuccessfully transmitted downlink data to the terminal,and wherein the downlink data delivery information further includes atleast one of information on a successfully delivered packet orinformation on a lost packet.
 7. The method of claim 5, wherein thefirst message includes packet data convergence protocol (PDCP) sequencenumber (SN) information for a first PDCP PDU, which is a first missingacknowledgement after a last PDCP PDU for which transmission has beenconfirmed.
 8. A method by a target distributed unit (DU) of basestation, the method comprising: transmitting, to a central unit (CU) ofthe base station, if a first message related to a handover istransmitted from the CU of the base station to a source distributed unit(DU) of the base station and a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal is transmitted from the source DU to the CU, athird message based on a random access procedure of the terminal towardthe target DU; and in response to a reception of the third message atthe CU, receiving downlink data from the CU based on the downlink datadelivery information, before transmitting a fourth message associatedwith a radio resource control (RRC) reconfiguration complete based onthe random access procedure of the terminal to the CU.
 9. The method ofclaim 8, wherein the downlink data delivery information includesinformation on unsuccessfully transmitted downlink data to the terminal,and wherein the downlink data delivery information further includes atleast one of information on a successfully delivered packet orinformation on a lost packet.
 10. The method of claim 8, furthercomprising transmitting the received downlink data to the terminal,before an RRC connection reconfiguration is completed.
 11. A centralunit (CU) of a base station, the CU comprising: a transceiver; and acontroller, which is configured to control the transceiver to: transmit,to a source distributed unit (DU) of the base station, a first messagerelated to a handover, receive, from the source DU, a second message fordownlink data delivery information related to a packet data unit (PDU)transmitted from the source DU to a terminal, receive, from a target DUof the base station, a third message based on a random access procedureof the terminal toward the target DU, and in response to receiving thethird message, transmit downlink data to the target DU based on thedownlink data delivery information; before receiving a fourth messageassociated with a radio resource control (RRC) reconfiguration completebased on the random access procedure of the terminal from the target DU.12. The CU of claim 11, wherein the downlink data delivery informationincludes information on unsuccessfully transmitted downlink data to theterminal.
 13. The CU of claim 11, wherein the downlink data deliveryinformation includes at least one of information on a successfullydelivered packet or information on a lost packet.
 14. CU of claim 11,wherein the first message includes packet data convergence protocol(PDCP) sequence number (SN) information for a first PDCP PDU, which is afirst missing acknowledgement after a last PDCP PDU for whichtransmission has been confirmed.
 15. A source distributed unit (DU) of abase station, the source DU comprising: a transceiver; and a controller,which is configured to control the transceiver to: receive, from acentral unit (CU) of the base station, a first message related to ahandover, and transmit, to the CU, a second message for downlink datadelivery information related to a packet data unit (PDU) transmitted toa terminal, wherein a third message is transmitted from a target DU ofthe base station to the CU based on a random access procedure of theterminal toward the target DU, and wherein in response to a reception ofthe third message at the CU, downlink data is transmitted from the CU tothe target DU, based on the downlink data delivery information, before afourth message associated with a radio resource control (RRC)reconfiguration complete based on the random access procedure of theterminal is received at the CU.
 16. The source DU of claim 15, whereinthe downlink data delivery information includes information onunsuccessfully transmitted downlink data to the terminal, and whereinthe downlink data delivery information further includes at least one ofinformation on a successfully delivered packet or information on a lostpacket.
 17. The source DU of claim 15, wherein the first messageincludes packet data convergence protocol (PDCP) sequence number (SN)information for a first PDCP PDU, which is a first missingacknowledgement after a last PDCP PDU for which transmission has beenconfirmed.
 18. A target distributed unit (DU) of a base station, thetarget DU comprising: a transceiver; and a controller, which isconfigured to control the transceiver to: transmit, to a central unit(CU) of the base station, if a first message related to a handover istransmitted from the CU of the base station to a source DU of the basestation and a second message for downlink data delivery informationrelated to a packet data unit (PDU) transmitted from the source DU to aterminal is transmitted from the source DU to the CU, a third messagebased on a random access procedure of the terminal toward the target DU,and in response to a reception of the third message at the CU, receivedownlink data from the CU based on the downlink data deliveryinformation, before transmitting a fourth message associated with aradio resource control (RRC) reconfiguration complete based on therandom access procedure of the terminal to the CU.
 19. The target DU ofclaim 18, wherein the downlink data delivery information includesinformation on unsuccessfully transmitted downlink data to the terminal,and wherein the downlink data delivery information further includes atleast one of information on a successfully delivered packet orinformation on a lost pack.
 20. The target DU of claim 18, wherein thecontroller is further configured to control the transceiver to transmitthe received downlink data to the terminal, before an RRC connectionreconfiguration is completed.
 21. A method by a central unit (CU) of abase station, the method comprising: transmitting, to a sourcedistributed unit (DU) of the base station, a first message related to ahandover; receiving, from the source DU, a second message for downlinkdata delivery information related to a packet data unit (PDU)transmitted from the source DU to a terminal; receiving, from a targetDU of the base station, a third message based on a random accessprocedure of the terminal toward the target DU; and transmitting, to thetarget DU, downlink data based on the downlink data delivery informationafter receiving the third message.
 22. A method by a target distributedunit (DU) of a base station, the method comprising: transmitting, to acentral unit (CU) of the base station, if a first message related to ahandover is transmitted from the CU of the base station to a source DUof the base station and a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal is transmitted from the source DU to the CU, athird message based on a random access procedure of the terminal towardthe target DU; and receiving, from the CU, downlink data based on thedownlink data delivery information after a reception of the thirdmessage at the CU.
 23. A central unit (CU) of a base station, the CUcomprising: a transceiver; and a controller, which is configured tocontrol the transceiver to: transmit, to a source distributed unit (DU)of the base station, a first message related to a handover, receive,from the source DU, a second message for downlink data deliveryinformation related to a packet data unit (PDU) transmitted from thesource DU to a terminal, receive, from a target DU of the base station,a third message based on a random access procedure of the terminaltoward the target DU, and transmit, to the target DU, downlink databased on the downlink data delivery information after a reception of thethird message.
 24. A target distributed unit (DU) of a base station, thetarget DU comprising: a transceiver; and a controller, which isconfigured to control the transceiver to: transmit, to a central unit(CU) of the base station, if a first message related to a handover istransmitted from the CU of the base station to a source DU of the basestation and a second message for downlink data delivery informationrelated to a packet data unit (PDU) transmitted from the source DU to aterminal is transmitted from the source DU to the CU, a third messagebased on a random access procedure of the terminal toward the target DU,and receive, from the CU, downlink data based on the downlink datadelivery information after a reception of the third message at the CU.